In general, in a spectrophotometer, monochromatic light having a specific wavelength is extracted by introducing light emitted from a light source into a light separator, and this monochromatic light is cast onto or into a sample as measurement light. The wavelength of the light which interacts with the sample is specific to the substances contained in the sample. Accordingly, detecting the light after the interaction (e.g. transmitted, reflected, scattered or fluorescent light) with a photodetector enables quantitative and/or qualitative determination of the sample based on the detection signal, the wavelength of the cast light and other information.
For example, in a measurement of a sample using an ultraviolet-visible spectrophotometer, light emitted from a light source having a continuous spectrum over a comparatively broad range of wavelengths, such as a halogen lamp or deuterium lamp, is separated by a light separator, and measurement light having a preset wavelength is cast into a sample held in a sample cell. The light generated from the sample is detected by a photodetector. The thereby generated detection signal is sampled by an A/D converter and converted into digital data with a preset period. The obtained digital data are sent to a data processing system composed of a personal computer or similar device. A predetermined kind of data processing (e.g. creation of an absorption spectrum) is performed by the data processing system.
In the A/D converter, using a shorter sampling period enables a more precise digital conversion of analogue signals received from the photodetector. However, a problem exists in that the data size increases. Accordingly, taking into account the peak width of a measured waveform produced by the spectrophotometer, the sampling period is set at the largest possible value within a range where the waveform can be A/D converted with adequate precision (e.g. at approximately 10 ms for an ultraviolet-visible spectrophotometer used as a detector for a liquid chromatograph).
In the previously described type of ultraviolet-visible spectrophotometer, the setting of the wavelength of the measurement light is achieved by adjusting the rotational angle of the diffraction grating provided in the light separator. Since the correctness of the set wavelength of the light separator significantly affects the accuracy of the spectral analysis, it is necessary to regularly validate the “wavelength correctness.” In a conventional process of validating the wavelength correctness of a light separator, light generated by a light source having an emission line at a known wavelength is introduced into the light separator, and the light exiting from the light separator is detected with a photodetector while the wavelength of light extracted by the light separator is continuously varied. Then, a spectrum with the horizontal axis indicating the wavelength and the vertical axis indicating the intensity of the received light is created, and an error of the set wavelength with respect to the true wavelength is calculated by comparing the peak position of the emission line on the spectrum and the true (theoretical) wavelength value of the emission line. This error represents the accuracy of the “wavelength correctness” in the light separator.
To guarantee the wavelength correctness over the entire measurement wavelength range of the spectrophotometer by the validation of the wavelength correctness, it is desirable to perform the previously described peak-position determination of the emission line at a plurality of points which are separated as widely as possible within the measurement wavelength range. In general, an ultraviolet-visible spectrophotometer is provided with a halogen lamp (for visible region) and a deuterium lamp (for ultraviolet region) as the light sources for sample measurement, allowing the two lamps to be selected according to the set wavelength. Among these light sources, the halogen lamp has no emission line within a wavelength region suitable for the validation. Therefore, normally, a low-pressure mercury lamp is additionally provided as a light source for validation (for example, see Patent Literature 1), and the validation of the wavelength correctness is performed using an emission line (253.7 nm) of this low-pressure mercury lamp and an emission line (656.1 nm) of the deuterium lamp.